Transformer cores consisting of a plurality of steel laminations are coated on both sides with a first layer of inorganic insulation coating followed sometimes by a layer of an insulating enamel to prevent eddy current transport between the laminations during transformer operation and substantially reduce core eddy loss that would otherwise result.
The presence of the resistive coating between the laminations is such that when a plurality of laminations are stacked together, the stacked core can act as a capacitor bank. In ordinary transformer operation at 60 cycle power frequency, there is little or no current transport occurring between the individual laminations due to the presence of the insulating coatings employed. Upon the occurrence of a high voltage surge condition, however, the transformer core laminations charge in a manner similar to the charging of a series connected capacitor bank. In order to prevent large charges from remaining on the transformer core during normal and surge conditions, a ground lead is interposed between two of the laminations to provide an electrical path to the ground. The insulating coatings, however prevent the ready transfer of the charge current out through the laminations to the ground lead connections. This has the effect of increasing the lamination-to-lamination voltage to above the breakdown strength of insulation.
The purpose of this invention is to provide methods and materials for coating the transformer core laminations and edges with a substance that has good resistance properties under normal operating conditions and readily conducts charge currents to adjacent laminations upon the occurrence of an impulse voltage. Excessive interlaminar potential differences and danger of breakdowns are therefore avoided.